Many types of electrochemical cells utilize a liquid ionically conductive medium to support electrochemical reactions within the cell. Electrochemical cells may utilize an air breathing oxidant electrode coupled to a fuel electrode, comprising any suitable fuel. For example, a metal-air electrochemical cell system may comprise a plurality of cells, each having a fuel electrode serving as an anode at which metal fuel is oxidized, and an air breathing oxidant reduction electrode at which oxygen from ambient air is reduced. The liquid ionically conductive medium in such cells may communicate the oxidized/reduced ions between the electrodes.
In various liquid ionically conductive mediums, evaporation, electrolysis (e.g. decomposing water on recharge) or other loss of moisture from the ionically conductive medium, may be detrimental to the electrochemical cell. For example, salting of the ionically conductive medium may clog an oxidant electrode of the electrochemical cell, reducing its performance or in extreme cases, result in loss of functionality. Such salting or other failures may occur, for example, where an air-side of the oxidant electrode, or a portion thereof, is excessively dry. Additionally, a decrease in water content in the ionically conductive medium may decrease the medium's solvating capacity, i.e., its ability to dissolve solutes, or increase the percentage concentration of solutes in the medium.
Accordingly, the disclosure in the present application endeavors to facilitate maintaining desired water content in the electrochemical cell, in addition to controlling humidity associated with an air breathing oxidant electrode thereof.
While the foremost example of the present disclosure involves electrochemical cells comprising air-breathing electrodes, this is not intended to be limiting in any way. The invention may be employed in any electrochemical cell comprising a liquid ionically conductive medium. For example, aqueous flooded batteries (e.g. lead-acid, Ni—Cd) in a wide range of capacities and sizes are known in the art. Often these are vented cells comprising a vent or low pressure release valve to release oxygen and hydrogen gases formed upon decomposition of water due to electrolysis. The electrolyte lost (e.g. by evaporation and/or electrolysis) must be periodically replaced through routine maintenance. Various means of replenishing the cells with water are known in the art including manual addition, high pressure injection manual addition and single point battery watering systems. Single point battery watering systems comprise a liquid water reservoir or tank connected via tubing to a water valve assembly associated with each electrochemical cell. Such battery watering systems are well-known in the art, for example U.S. Pat. Nos. or Application Publication Nos. 4,386,141; 5,284,176; 5,453,334; 6,164,309; 6,786,226; 7,040,431 7,556,056; 2006/0281000 and 2011/0236730. Each valve assembly may comprise a mechanical level-control valve, commonly float valves, to shut off the flow of water once a predetermined electrolyte level is reached. Often, these mechanical valves may fail or stick and liquid water flows unimpeded. This is especially problematic for aqueous alkaline cells (e.g. Ni—Cd) because dried salt often clogs the valves. To avoid this, development of valves operating via high pressure bursts are known in the art. However, such high-pressure systems typically require both power and long-term reliability of a pump.
Accordingly, the disclosure in the present application provides for a passive water management system to maintain water content in any electrochemical cell comprising a liquid ionically conductive medium without the need for pumps, liquid water reservoirs and mechanical level-control valves.